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Climate Change – Killing Trees in Our Neighbourhood

Well, anyone who says that temperate countries could do with a little warming ought to read this study. Trees are dying at double the usual rate in the last 17 years.

Warmer temperatures have dramatically increased the rate at which trees in old-growth forests are dying in parts of British Columbia and the western United States, a study says.

The study, to be published in Friday's edition of the journal Science, found that mortality rates for trees in the old-growth plots in the Pacific Northwest — including parts of southern British Columbia — had doubled in 17 years.

Forests in California and other states had less dramatic numbers. The interior states — like Arizona, Colorado and Idaho — had tree mortality rates that doubled every 29 years. The mortality rates change incrementally every year, the researchers say.

“We may only be talking about an annual tree mortality rate changing from 1 per cent a year to 2 per cent a year, an extra tree here and there,” study author Mark Harmon, professor of forest ecology at Oregon State University, said in a statement.

“But over time a lot of small numbers can add up. The ultimate implications for our forests and environment are huge.”

The increases in mortality rates are replicated across all trees at every elevation, regardless of species or size.

The study, which the researchers say is the first of its kind on temperate forests, gathered data on 76 long-term forest plots over a 50-year period for analysis. All of the forest areas studied were at least 200 years old, although individual trees varied in age and size.

The study controlled for all other variables including the infamous pine beetle, and found that temperature increase was to blame. Why? A 1ºC rise in temperature results in less snow, longer summer and increased drought stress. The effects on any one individual tree would not be significant, but if you look at an entire population, these stresses caused a doubling of mortality.